Coated water soluble inorganic oxidizers



United States Patent 3,551,222 COATED WATER SOLUBLE INORGANIC OXIDIZERS Martin H. Kaufman, John D. ODrohinak, and John W. Carroz, China Lake, Calif., assignors to the United States of America as represented by the Secretary of the Navy N0 Drawing. Filed Mar. 6, 1967, Ser. No. 621,732

Int. Cl. C06b 19/02 U.S. Cl. 1497 10 Claims ABSTRACT OF THE DISCLOSURE Ammonium perchlorate, potassium perchlorate and potassium nitrate coated with a silane or chrome complex for use in composite propellant formulations whereby improved bonding between the binder and oxidizer results in greatly increased elongations.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to coated water soluble inorganic oxidizers and, more particularly, to ammonium perchlorate, potassium perchlorate and potassium nitrate coated with a silane or chrome complex.

In the solid propellant field composite propellant compositions containing an inorganic oxidizer and a polymeric binder as a fuel are among the types employed to supply thrust for rocket engines. Improved oxydizerbinder systems are of primary concern. The present invention is for an improved propellant oxidizer-binder system wherein the binder-solids adhesive strength is about as strong as the cured binder cohesive strength.

It is therefore an object of the present invention to provide a coated inorganic oxidizer for use in composite rocket propellant formulations.

Another object is to provide a coated inorganic oxidizer which when used in propellant formulations aids in reducing the environmental effect of humidity on the long range physical properties of the propellant grain.

Yet another object is to provide a simple method for coating water soluble oxidizer ingredients for use in pyrotechnics, explosives and propellants.

Still another object is to provide an oxidizer-binder system wherein improved bonding between the binder and oxidizer results in increased propellant elongations.

Other objects, features and many of the attendant advantages of this invention will become readily appreicated as the same become better understood by reference to the following detailed description.

In accordance with the present invention silane or chrome complexes diluted with water and hydrolyzed with ammonium hydroxide and a weighed amount of the water soluble oxidizers selected from the group consisting of ammonium perchlorate, potassium perchlorate and potassium nitrate were thoroughly mixed together. The resulting mixture was dried in a hot air stream at 190- 210 F. The hot air stream keeps the inorganic oxidizer particles essentially separate from each other and provides a very rapid evaporation of the water and/ or other diluent such as alcohol. The dry coated oxidizer was then heated for from 16-20 hours in an oven maintained at 85 to 100 C. to assure complete polymerization of the coating around the individual oxidizer crystals. The coating present is in an amount ranging from 0.025 to 1.0 part by weight per 100 parts by weight inorganic oxidizer. When the coated oxidizer was blended into a hydrocarbon binder, carboxyterminated polybutadiene (CTPB) in this instance, curing of the propellant was not afi'ected, nor did aging appear to affect the elfectiveness of the coating. The physical properties of the propellant were improved due to the coated oxidizer because of improved adhesion between the binder and the oxidizer.

The nature or mechanism of the present process is not known. The silane and chrome complexes were found to contain groupings which were widely different in polarity. One of the groupings was found to be very polar so as to promote adhesion to the ionic ammonium perchlorate and the other grouping was very similar to the binder molecular structure used in the composite propellant system. It is known that ammonium perchlorate is ionic and on drying holds on to the last trace of water tenaciously. Now, since the properties of water result from its hydroxyl (OH) components, it was hypothesized that one end of the coating complex or agent should also be a hydroxyl, and the other end would be mainly hydrocarbon. If a different binder type were used, the coating complex would be different in order to conform to the binder.

Two similar materials which were found to have the required structures were alkyloxy silanes and Werner type chrome complexes. The pertinent reactions of both are quite similar, i.e., hydrolysis to hydroxyl groups followed by polymerization on heating.

The chrome complexes are best represented as follows:

wherein R is a radical derived from an organic acid selected from the group consisting of gallic, sorbic, linoleic, methyacrylic, cyanoacetic, stearic and myristic. The Werner-type chromium complexes of carboxylic acids used herein were developed from the reaction products of chromyl chloride and various organic acids. Typical complexes are the Quilon" chrome complexes, a registered trade name product comprising for example, a chromium complex of stearic acid having the formula uHu and Volan another registered trade name product comprising the chromium complex of methycrylic acid. The chemistry of chromium complexes, methods of manufacture, application and various uses are discussed in Advances in Chemistry Series (1959), No. 23, Chromium Complexes by F. B. Hauserman, pages 338-356.

The alkyloxy silanes are represented by the following formula wherein R is an alkyl group and R represents a large number of long chain radicals or groups. The silanes used herein are known commercially as Silane 4086 which is 3,4-epoxycyclohexyltrimethoxysilane and Silane 1100 which is aminopropyltriethoxysilane.

The structure of R is only limited by practical synthetic techniques and may be represented by an extremely large number of groups or structures. In the case of the chrome complexes they represent any group alpha to a carboxylic acid.

Under similar conditions the chromium complexes and TABLE [.THE EFFECT OF COATING AGENT CONCEN- '1 RATION ()N PROPELLANT C URE RATE the silanes used herein hydrolyze and on heating polymfig i M 1 1 21 mm El l n I 1501' )d (3 [N ll US, lull d onga- 10 a ('1' enze- The gel-Era] reactlon Shown as follows' 5 chrome psi. "lcnsile maximum t'lon at days complex, (10 pcrstrength, tensile, rupture, at 180 F, percent cent) p.s.i. pcrccnt percent percent 1,079 108 12 17 '14 1,010 103 12 14 711 955 in 2s 73 1,105 112 20 23 77 II; If H2U 1, 052 10s 32 74 C C 1 4 H20 Aging does not appear to affect the effectiveness of the &8 heat coating. Table 11 below using Volan M for a coating, Cr Cr Cr 01' V which is the chromium complex of methacrylic acid, gives H OH some experimental data on aging characteristics.

II II TABLE 1I.'II1E EFFECT OF AGING 0N COATING I? EFFECTIVENESS C n v i Aged at 180 1 LBKS 0 1 2 3 0 0 20 Tensile strength of control, p.s.i 163 168 170 170 l l Tensile strength of coated, psi. 160 169 179 183 Elongation of control, percent 8. 2 7. 1 6. 5 5. 2 l Elongation of coated, percent. 14. D 11.5 11.0 9. (1 H II II for bonding to AP or The effectiveness of ammonium perchlorate (AP) R R coated with silane or chrome complexes in an unopti- H20 1 heat I, mized composite propellant was examined. The physical I I O7S R 0 OR HO OH 0 property data are tabulated in Table III. The propellant 0H 0H used consisted of carboxyterminated polybutadiene poly- M bonding 30 mer and an imine curative. The formulations were cured WAP for one week at 135 F.

()R BONDING Al TO Concentration, grams Tensile Elongation Elongation s0lun, 100 g. Molecules strength, at maximum at rupture Coating agent Al 10%, ps1. p.s.i. TS, percent percent Control 2, (180 H4 13 16 Volan E ll. 5 1, J4Up .17 211 3 Volan l[.. 0.5 1, 760 87 3O 3 Volan Z (l. 5 1, 960 Slti 2:1,; Volan Zp 0. i3 2, 070 15 32 30 Volan M l). 5 1, 810 94 25 28 Volan M. 0. T3 2, D00 92 28 38 Volan X. D. 5 2, 420 102 23 22 Volan X 0. T3 1, J30 JD 27 37 Quilon S. l). 5 1, 920 92 2|) 21 Quilon S. (1.73 1, 630 86. 5 22. 5 24 Quilon M D. 5 2, Bill) .16 2f) 28 Quilon M D. 73 1,670 83 24 36 Silanc 4086 0. 5 2, 720 103 29 30 Silanc 1100 0. 5 2, 660 104 23 l Letter designate chain on the chromyl complex (lcrivcd from organic acid:

Volan E Gallic.

Volan II orbie.

Volan Z= Linoleic.

Volan M=l\lethacrylic.

Volan X =0 ynnoacotic.

Quilon S Staarie.

Quilon M=Myristie. b 3,4-epoxycycl0hexyltriincthoxysilanc. Aininopropyltriethoxysilane.

The coating solutions contain about 5 to 30% solids. The actual coatings range from 0.025-0.25% of the ammonium perchlorate.

The coating complex does not appear to affect propellant cure. Table I below illustrates the effect of the chrome complex on propellant cure.

The effects of increased propellant solids loading and high temperature aging (180 F.) on the adhesion properties were determined by using Volan E coated AP as a representative example (Table IV). Data shows that the coating is effective at solids loadings of 85% as well as and that it remains effective under high temperature aging even though the binder used in the propellants degrades.

TAB LE IV.-EFFECT OF SOLIDS LOADING AND HIGH TEMPERATURE AGING ON PROPELLANTS UTILIZING COATED AP (EX-868 CURE) Physical properties were determined at different temperatures on propellants which contained either coated or uncoated AP. The data shows effective adhesion at +77 F. and +l65 F. There is the possibility that cohesive failure occurred in the binder at 65 F. (Table V).

TABLE V.COATING EFFECTIVENESS 0N PROPELLAN'I PHYSICAL PROPERTIES MEASURED AT DIFFERENT TEMPERATURE (T110868 CURE) To gain further evidence that the bond between the oxidizer and the binder was strengthened samples of Volan E coated ammonium perchlorate crystals and uncoated ammonium perchlorate crystals were placed into the binder side by side. The material was then cured. The cured material was then stressed using an Instron tensile machine and periodic photographs were taken. They showed that the binder pulled away (dewetting) from the uncoated ammonium perchlorate crystals, but the bond between the Volan E coated ammonium perchlorate crystals and the binder remained intact.

The following Table VI gives the coating effectiveness on propellant physical properties wherein the coating was Volan E and the oxidizers were potassium perchlorate and potassium nitrate.

TABLE V[.COA'IING EFFECTIVENESS ON PROPELLANT IlIYSlCAL PROPERTIES (THE PROPELLAN'I CON- SISTEI) ESSENTIALLY OF 20% BINDER, 16% ALUMINUM AND 64% OXIDIZER) Modulus Tensile Elongation Coating (p.s.l.) (p.s.l.) (percent) Control (No coating) 6. 700 163 4 Volan E coated potassium nitrate. 6,300 194 9. 2

Control (No coating) 5, 080 144 5. 6 Volan E coated potassium perchlorate- 4, 640 183 9. 9

TABLE VII.EFFECT OF IIYDROLYSIS VARIABLES ON PHYSICAL PROPERTIES A P saturated Elongation Solution, AP water in Modulus, Tensile at maximum contact time, coating (10%), strength, tensile, hours solution, cc. p.s.l. psi. percent 40 1, 170 82 42 Control 1,320 81 18 Within the limits of the test, variation in contact time has broad limits, as well as volume of water used per gram of ammonium perchlorate (AP).

In order to determine whether or not the AP saturated solution was aflecting the AP particle size distribution a screen analysis was run on random samples of both coated and uncoated AP (Table VII).

The data indicates negligible alteration of the particle size distributions for the AP that was tested.

By this invention the elongations of composite propellants have significantly increased without appreciable lessening of tensile strength. Through improved propellant elongations by means of the coated inorganic oxidizers,

ammonium perchlorate, potassium perchlorate and potassium nitrate it is now possible to achieve higher loadings while still maintaining acceptable physical properties. In addition strength may be improved while still maintaining acceptable elongation.

The technique of application and of drying the coating complex on the oxidizer particles is critical and involves careful controls. The complex was diluted with a small quantity of water or alcohol, then hydrolyzed with dilute ammonium hydroxide. Additional water and/or alcohol was added to raise the volume of the solution to just wet the amount of ammonium perchlorate to be coated. The solution was then added to the ammonium perchlorate which was then thoroughly mixed with a soft spatula. The next step was to dry the product, Two procedures were used: (1) glass rotary evaporation and (2) hot air stream (fluidized bed). The latter gives consistently good results.

In the rotary evaporator the ammonium perchlorate very often stuck to the glass wall, moreover, particle-particle contact prior to hard cure of the agent may actually rub off the coating.

In the fluidized bed a hot (HO-210 F.) air stream keeps the oxidizer particles essentially separate from each other and provides a very rapid evaporation of the water and/or alcohol.

In order to insure complete polymerization of the coating around the individual oxidizer particles the freshly dried materials were heated for at least 16 hours in an oven maintained at to C. Results using the fluidized drying technique are presented in Table VI below. The coating materials and process used were as follows:

(1) 3.00 g. Volan H (2) 20 or 40 ml. of AP saturated water (3) Addition of 6.6 g. of 1% NH OH with rapid stirring (4) Pour resulting solution onto 600 g. of AP and mix with a soft spatula (5) Dry in fluidized bed for 10 minutes at F.

(6) Post dry in steam oven for 18 hours at 190 F.

Humidity is detrimental to composite propellant grains due mainly to the presence of the water soluble oxidizers. In the present invention the hydrocarbon chain on the coating complex will no doubt act as an effective barrier to water molecules. The coating agents, acting as a barrier between the oxidizer and binder, will probably also eliminate the deleterious effects of the oxidizer on the overall curing and aging process.

Although the process described herein produces a single coating on a water soluble crystal of a silane or chrome complex, coating the cured crystals with yet another layer of the same complex or another complex can be achieved by the same procedure. For example, ammonium perchlorate coated with a chrome complex may then be coated with the silane complex and cured. It is probable that the double coating may effect improved characteristics such as water impermeability.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. Coated inorganic oxidizers selected from the group consisting of ammonium perchlorate, potassium perchlorate and potassium nitrate, wherein the coating is a chromium complex selected from the group represented by 0 -L l 0 Gr *0 0 o 11 ll 11 wherein R is a radical derived from a member selected from the group of organic acids consisting of gallic, sorbic, linoleic, stearic, methacrylic, cyanoacetic and myristic and wherein the coating is present in an amount ranging from 0.025 to 1 part by weight per 100 parts by weight inorganic oxidizer.

2. The product of claim 1 wherein oxidizer is ammonium perchlorate and the coating is the chromium com plex of methacrylic acid.

3. The product of claim 2 wherein the coating is the chromium complex of gallic acid.

4. The product of claim 2 wherein the coating is the chromium complex of sorbic acid.

5. The product of claim 2 wherein the coating is the chromium complex of linoleic acid.

and

6. The product of claim 2 wherein the coating is the chromium complex of cyanoacetic acid 7. The product of claim 2 wherein the coating is the chromium complex of stearic acid.

8. The product of claim 2 wherein the coating is the chromium complex of myristic acid.

9. The product of claim 1 wherein the oxidizer is potassium perchlorate and the coating is the chromium complex of gallic acid.

10. The product of claim 1 wherein the oxidizer is potassium nitrate and the coating is the chromium complex of gallic acid.

References Cited UNITED STATES PATENTS 3,058,858 10/ l 962 Batchelder et a1. l4919 3,190,775 6/1965 Ender 149-7X 3,190,776 6/1965 Ender 149-7X LELAND A. SEBASTIAN, Primary Examiner US. Cl. X.R. 

